I think you need to brush up on your theory:-
the loudest is at 50% duty cycle (only on half the time). Which at 5 volts, is like sending only 2.5v to the speaker. With toneAC, we're sending out of phase signals on two pins. So in effect, the speaker is getting 5 volts instead of 2.5, making it nearly twice as loud.
This is wrong.
What is happening is that the standard tone library you are sending 5V in a single ended drive. With what you are doing you are sending +/- 5V in a push pull arrangement so in effect boosting the output to 10V. Which is why it is twice as loud.